PHP

Locking Files

When several people try to open the file at the same time, there is a problem. So the solution is to temporarily lock the file while PHP is opening and writing to it.

Use flock()

Lock	Meaning
LOCK_SH	Shared lock for reading purposes
LOCK_EX	Exclusive lock for writing purposes
LOCK_UN	Release of a lock
LOCK_NB	Non-blocking lock

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="EN" lang="EN">
<head>
<title>Locking Files</title>
<meta Name="Author" Content="Hann So">
</head>
<body>
<p>
<?php

if (isset($_POST['submit'])) {
		process_form();
	}
	else {
		display_form();// display form for the first time
	}

function display_form() {
	echo <<<HTML
	<h2>Feedback Form</h2>
	<form action = "$_SERVER[SCRIPT_NAME]" method="post">
	Filename:
	<input type="text" name="file" size="50" value="data/feedback.txt" />
	<br />
	Comments:
	<textarea name="comments" rows="5" cols=""50">here are my comments.</textarea>
	<br />
	<input type="submit" name="submit" value="Submit" />
	</form>
HTML;
}

function process_form() {

	$file = "$_POST[file]";
	$data = stripslashes($_POST['comments']);

	// open the file for reading
	if ($fh = fopen("$file", 'ab')) {
		// lock for writing
		flock($fh, LOCK_EX);
		// write the data
		fwrite($fh, "$data\n");
		// unlock after writing
		flock($fh, LOCK_UN);
		// close the file
		fclose($fh);
		echo "<p>Your feedback has been stored.</p>";
	}
	else {
		echo "<p>Your feedback could not be stored.</p>";
	}

	echo "<p><a href=\"$_SERVER[SCRIPT_NAME]\">Try again?</a></p>\n";
	echo "<p><a href=\"example_r.php\">
	Do you want to read from the file?</a></p>\n";
}

?>
</p>
</body>
</html>

View the effect